Porous polymeric materials with interconnected porosity are receiving great attention recently due to their wide range of potential applications. These potential applications touch various fields such as: biomedical and pharmaceutical applications [tissue engineering, extra-corporeal devices such as those that are connected to the body to assist in surgery or dialysis, implantable medical devices that serve for example as substitutes for blood vessels, intra-ocular lenses, bio-sensing electrodes, catheters, and drug and gene delivery (for example as substrate for controlled release)], materials for chromatographic and other separation techniques (such as filtration, dialysis and osmosis), materials for diagnostic devices such as lateral flow devices, flow through devices and other immunoassay devices, catalysis of chemical and biochemical reactions, materials for conductivity applications and the list goes on.
In the sphere of tissue engineering, porous materials have particular use as polymeric scaffolding for cell tissue engineering. Interconnectivity of the pore network is essential for tissue ingrowth, vascularization and diffusion of nutrients. Controlled pore size distribution and large void volumes are other important features.
Much scientific effort is focused on biodegrable polymers such as poly(alpha esters) such as poly(lactic acid), poly(glycolic acid) and their copolymers. Such materials are approved for human clinical use in many jurisdictions.
Five different techniques to fabricate interconnected porous polymeric substrates for tissue engineering applications have been developed: textile process (ref 1), particulate leaching (ref 2), gas foaming (ref 3), thermally induced phase separation (refs 4 and 5) and hydrogel cross-linking (ref 6). However, every one of these approaches suffers from severe limitations including: low levels of interconnectivity; low void volume; structural fragility; difficulties in functionalization; poor control of pore size and distribution; and difficulties in obtaining reproducible porosities and large 3D articles.
Recently, an approach to preparing microporous materials, based on the melt-blending of two immiscible polymers, has been described (refs 7-10). It was shown that selective extraction of one of two components in a co-continuous structure can result in a single component structure of fully interconnected porosity with finely controlled pore size, porosity and morphology. This technique can generate porosities with high surface areas and pore diameters ranging from as low as about 100 nanometers upwards to hundreds of microns (refs 8-10). On the other hand this approach is limited in the void volume that can be attained and it can be used to prepare only relatively low void volume substrates (75%).
In an entirely different field of scientific activity, self-assembly via layer-by-layer (LBL) deposition is gaining significant attention. LBL deposition is a simple and effective approach to deposit ultrathin and uniformly assembled molecular layers on surfaces (refs 11 and 12). In that approach, polyelectrolytes are adsorbed on an oppositely charged surface, reversing the surface charge and leaving it primed for the next deposition cycle (refs 13 and 14). The resulting structure is multilayered with each molecular layer composed of oppositely charged polyelectrolytes. The thickness of each layer can be closely controlled through the salinity of the polyelectrolyte solution (refs 13 and 14), the quality of the solvent (ref 15) and the charge density on the polyion chain (ref 16). Deposition time, polyelectrolyte concentration and molecular weight are known to be less important parameters (ref 12). Originally exclusively investigated on macroscopically flat surfaces, this technique has been recently used to fabricate hollow spheres by assembling multilayered films onto colloidal particles and subsequently removing the core (refs 17 and 18).
To date, however, no work has been reported on the use of LBL techniques to create 3-dimensional, fully interconnected substrates.